Delayed-Choice Experiments

Some quantum optics researchers in France very recently reported an experiment in which photons went through the apparatus one at a time. Each photon was directed along two paths, with horizontal polarization in one path and vertical polarization in the other. The experimenters then had the option of making the analysis with horizontal or vertical polarizers, revealing which path the photon went, or with diagonal polarizers, revealing fringes and anti-fringes. Does all this sound familiar? The curious feature of this particular experiment is that the choice of which kind of polarizer to use happened after the photon had already split into the two paths. The results showed that the timing of the decision didn't affect the results at all. Indeed the results agreed with those of the simple experiment described in the main article.
Reference for this experiment: V. Jacques, et al. Science 315, 966 (2007).

As we mentioned in the main article, it may seem as though you could use this delayed-choice feature to send signals instantaneously. To see how, consider the set-up described there to explain how a quantum eraser works in general: You send particles through two slits so that they can interfere, but individual photons bounce off them near the slits, destroying the interference. By making a special kind of measurement of the photons, you can erase the which-slit information and restore interference.

To use this kind of set-up to try to transmit a signal to someone who is far away, you arrange for the potentially interfering particles to travel all the way to that recipient, and she can observe whether they form fringes or not when they arrive. Meanwhile, you have to keep the photons at the ready, perhaps by circulating them through long coils of optical fiber. Now when the recipient is about to receive a batch of particles, you can transmit a "0" to her by measuring the corresponding photons to determine which slit each particle went through (so she will see no interference with that batch of particles), or you can send a "1" by doing the special measurement that restores interference. Because of the delayed-choice effect, seemingly your choice of measurement will instantaneously determine what the recipient sees no matter how far away she is.

As with the apparent paradox discussed in the main article, this strategy is foiled because to see the fringes formed by the particles, your distant friend must first divide them into two groups to separate ones that form fringes from those that form anti-fringes. To do that division, however, she needs to know the results of your special photon measurements—essentially there are two types of erasing results, those corresponding to particle interference fringes, and those corresponding to particle anti-fringes—and that information is going to crawl its way to her no faster than the speed of light.